How to get consistent lamp strengths in cycles?
I'm not completely sure why the spot lamp is generally darker than the rest of them, and the point lamp slightly darker, but if you level the playing field a bit, they aren't all that different:


Set all sizes to the same value, 0.5 as well as the dimensions of the mesh light
Set the spot lamp's shape size (the cone) to 180 degrees




Now you can see that the mesh and area lamps are nearly identical, except for the area lamp being slightly brighter in the middle on the back wall.

The dimensions of the mesh light was an arbitrary guess, changing it will change the brightness since light is emitted from the surface of the mesh and more surface area means more light. This is different from the other lamps, where the size is purely to control the softness of the shadows and doesn't affect the brightness at all.